In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a "transparency" of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the "projecting" radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. "Long" or "soft" x-rays (a.k.a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG. ("Angstrom")) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning (e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015), available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration ("VLSI") involves chips with circuitry built to design rules of 0.25 .mu.m. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet ("UV") delineating radiation. "Deep UV" (wavelength range of .lambda.=0.3 .mu.m to 0.1 .mu.m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 .mu.m or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only serving several steppers.
Another source is the plasma x-ray source, which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet ("YAG") laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 .mu.m to 250 .mu.m spot, thereby heating a source material to, for example, 250,000 .degree. C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). A stepper employing a laser plasma source is relatively inexpensive and could be housed in existing facilities. It is expected that x-ray sources suitable for photolithography that provide bright, incoherent x-rays and that employ physics quite different from that of the laser plasma source will be developed.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term "proximity"), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength .lambda..ltoreq.16 .ANG. is required for 0.25 .mu.m or smaller patterning to limit diffraction at feature edges on the mask.
Use has been made of the synchrotron source in proximity printing. Consistent with traditional, highly demanding, scientific usage, proximity printing has been based on the usual small collection arc. Relatively small power resulting from the 10 mrad to 20 mrad arc of collection, together with the high-aspect ratio of the synchrotron emission light, has led to use of a scanning high-aspect ratio illumination field (rather than the use of a full-field imaging field).
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced because the mask does not have to be positioned within microns of the wafer as is the case for proximity printing. The cost of mask fabrication is considerably less because the features are larger. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) in bands at which multilayer coatings have been developed (i.e., .lambda.=13.4 nm, .lambda.=11.4 nm) allows the use of near-normal reflective optics. This in turn has lead to the development of lithography camera designs that are nearly diffraction limited over useable image fields. The resulting system is known as extreme UV("EUVL") lithography (a.k.a., soft x-ray projection lithography ("SXPL")).
A favored form of EUVL projection optics is the ringfield camera. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow arcuate fields of aberration correction located at a fixed radius as measured from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width defines a region in which features to be printed are sharply imaged. Outside this region, increasing residual astigmatism, distortion, and Petzval curvature at radii greater or smaller than the design radius reduce the image quality to an unacceptable level. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image and use of object:image size reduction of, for example, 4:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
Sweatt, U.S. Pat. No. 5,361,292, discloses a condenser that includes a series of aspheric mirrors on one side of a small, incoherent source of radiation. If the mirrors were continuously joined into a parent mirror, they would image the quasi point source into a ring image with a diameter of a few tens of centimeters at some distance, here some number of meters. Since only a relatively small arc (about 60 degrees) of the ring image is needed by the camera, the most efficient solution is to have about five 60 degrees beams, all of which are manipulated such that they all fall onto the same arc needed by the camera. Also, all of the beams must be aimed through the camera's virtual entrance pupil. These requirements are met in two steps.
First, the beams are individually rotated and translated, as necessary, using mirrors so that they overlap at the ringfield and pass through the real entrance pupil. The second step is to image this real entrance pupil into the camera's virtual entrance pupil using a powered imaging mirror. This places the corrected, combined images of the mirrors into the proper position for use by the camera. This system may be configured in a variety of ways.
The earliest ringfield EUVL cameras as exemplified by Jewell et al., U.S. Pat. No. 5,315,629, that are designed for printing large (25 mm.times.25 mm) chips had instantaneous fields of view with an average radius of 25 mm and a chord length of 25 mm. When this type of ringfield camera is employed with the condenser of U. S. Pat. No. 5,361,292 the angle of the chord is 60 degrees which fit the 5 off-axis segments of the aspheric mirror, each 60 degrees wide, that comprise the illuminator or collecting mirrors of the condenser. However, with improved camera designs that have roughly the same chord length but with a much larger, e.g., 52 mm, average radius of the ringfield, the angle of the chord is about 28 degrees, into which the six ring images created by the illuminator mirrors are fitted. Six times the 28 degrees is significantly less half of the 360 degrees available so that a condenser using the design presented in U.S. Pat. No. 5,361,292 would be fairly inefficient.
One of the major causes of nonuniformity of line width in wafers is nonuniforrnity of illumination at either the reticle or wafer. A related factor which influences the critical dimensions is the partial coherence illumination properties along the ring field; the partial coherence being the fraction of the pupil filled with radiation from the condenser. Thus, despite the advantages in condenser designs for projection photolithography, the art is in search of techniques for enhancing image quality by improving illumination uniformity and/or controlling the partial coherence.